[MERGE] deprecate find_previous_heads

[Robert Collins]

This patch deprecated find_previous_heads, using Graph.heads() instead,
which is just a renamed _filter_candidate_lca's (because it is in fact
performing a heads calculation).

This adds 1 second to a 3m10 userspace commit due AFAICT to the slightly
increased function calls per file. I plan to remove that in further low
level restructurings.

I also refactoring RootCommitBuilder to remove duplication with
CommitBuilder - there is a single extra function call per commit, so
nearly no overhead there.

-Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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